The monsoon forecast is looking average. The Climate Prediction Center (CPC) is forecasting neither above nor below normal chances of rain totals in Arizona for July, August, and September. These are the three wettest monsoon months for the area. Basically the forecast is a toss of the coin. However, there is a science behind why this is the forecast.
We start with Mexico moisture. When the forests of the Sierra Madre Mountains south of the border are green and healthy, they re-release moisture gathered from the early monsoon downpours. This helps the monsoon rains move north into Arizona by early July. That means drought south of the border is bad for the monsoon here in Arizona. If the forests of the Sierra Madre Mountains are stressed, the plants won't re-release as much moisture into the air, possibly delaying the start of the monsoon in Arizona. Below is the North American Drought Monitor. There are drought conditions south of the border but the situation is not severe or extreme.
But note that drought is especially strong in states like Kansas, Oklahoma, and Texas. This is actually good for the monsoon. Drought in these areas can strengthen the summer temperature difference between the hotter land and cooler sea. This helps to draw more tropical moisture inland.
Then there is the El Niño and La Niña phenomenon in the East Pacific Ocean. This can influence the monsoon in the same way drought in the mid-section of the United States can affect the monsoon. El Niño conditions are warmer than average sea temperatures, which can weaken the land/sea temperature contrast and possible weaken the monsoon. La Niña conditions are cooler than average sea temperatures, which can enhance the land/sea temperature contrast and therefore possibly be good for the monsoon. This summer there is no El Niño or La Niña.
Hurricanes and tropical storms off the west coast of Mexico can enhance the monsoon with surges of moisture. This year the National Hurricane Center is forecasting an average year for tropical action in this area. While the influence of the storms depend very much on the track of the individual storms, the overall forecast indicates these storms will not provide much more moisture than their usual contribution.
The start of the monsoon, can also be impacted by spring snow pack in the Rocky Mountains. Spring snow on top of the mountains can delay the heating of the land in this area, reducing the land/sea temperature contrast. Northern Colorado was hit hard by spring storms and this is where snow lingers at the high elevations.
Looking good for the start of the monsoon is a wave of tropical energy moving across the Pacific from west to east. This wave is known as the Madden-Julian Oscillation (MJO). As the MJO moves into place off the west coast of Latin America, it can increase tropical activity. With a wave moving into this area right now, there is a possibility the increased activity will increase the chances of an on-time start to the monsoon here in Arizona. (Generally the heavy downpours do not begin until around July 4th.)
As you have probably noticed by now, there is no one factor that influences the overall monsoon forecast. And as you have noticed the factors this year are both good and bad with some having little to no influence this year. Adding everything up, the forecast is for neither a good or bad year. Just an average year. But keep in mind a surge of tropical moisture and energy can rev up the monsoon for a brief period of time. In 2011, the monsoon ranked 8th on the Top 10 wettest monsoons for Tucson. However, nearly a third of the total rainfall at the Tucson International Airport, where official measurements are taken, came on one day in September.
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